Case of the Failing 'C' Batteries

Our plant manager dropped several files on my desk and told me that previous colleagues had not found a solution to a problem where C-cells were being sporadically rejected at final test. Now it was my turn to find the answer.

I spent some time reading through the files and noted that my predecessors had worked back from raw materials, through detail parts and subassemblies. The attempt to find the source of the problem was terminated at various times, which indicated that the problem had either disappeared or my predecessors had moved on to other jobs. Their thoroughness showed they were not able to identify anything unusual in their investigations, so I decided to work backwards from cell testing to the place they had stopped their investigations.

When the employees in the testing area went on their breaks, I examined the test machine. It consisted of a multi-station carousel somewhat like the chamber of a revolver into which cells were fed. The chamber carried the cells to terminals, which carried out a loaded volts test. If the cell passed, it was carried on to a station that rolled the cell down a plastic channel and a worker stored the cell in an insulated tray. If it failed, a gate opened and the cell dropped into a plastic bucket, and the worker would stack it in another tray as a rejection.

On impulse, I started up the machine and fed some rejected cells through the tester. Most of them passed. When I hand-tested them, they were verified as OK. I continued doing this until I had accumulated about a dozen “rejects.” During the exercise, I marked one location which had intermittently led to a rejected cell, and when I passed the dozen rejected cells through the machine, they all passed -- except the ones at the marked location.

I then took a batch of passed cells and manually cycled the tester until the cell at the marked location registered with the test terminals. Sometimes they passed, but most of the time they failed. I noticed that the failed condition had the cell sitting canted in the chamber. By manually reorienting the cell, it would pass the test.

By this time, the tool-room foreman had turned up because he noticed the machine was running during break time. I showed him what was happening. He disappeared and returned shortly with another carousel and had one of the mechanics replace the chamber. When the test workers returned from break and resumed testing, the reject level dropped dramatically.

Over time, a large number of rejected cells had ended up sitting in the warehouse. A quality audit revealed that many of them were OK when hand-tested but showed intermittent failures when machine-tested. Because the confidence level was low, the cells remained in quarantine. After the testing machine defect was identified and verified, we issued a directive to recycle all rejected C-cells in the warehouse for re-evaluation.

This entry was submitted by John Mitchell and edited by Rob Spiegel.

John Mitchell was self-employed through Mitchell Research. He worked mostly in aerospace design/liaison engineering with excursions into product/quality engineering on batteries, forensic engineering analysis, and Hovercraft. He is now retired and working on vertical axis wind turbine systems and small electric vehicles.

It is interesting that none of your predecessors seemed to have taken the time to check to see if the rejected batteries were actually rejects. There is an old saw about 'assuming'. The problem is not 'assuming'. The problem is not recognizing what assumptions you are making, and then not reviewing those assumptions to see if they are valid.

John, as Gelnn said, this is a good example of not making assumptions. Your predecessors all assumed that the test harness was fine. That is often where there is a problem. Again, check the whole chain starting from where the failure occured.

John, as Gelnn said, this is a good example of not making assumptions. Your predecessors all assumed that the test harness was fine. That is often where there is a problem. Again, check the whole chain starting from where the failure occured.

John, as Gelnn said, this is a good example of not making assumptions. Your predecessors all assumed that the test harness was fine. That is often where there is a problem. Again, check the whole chain starting from where the failure occured.

It's a common assumption; people tend to believe that components are bad. Even when you have evidence to the contrary, it's easier to blame a component than a process or a tool. This issue is magnified when you have a marginal design, one that only works when a component is at its nominal rather than one end of the tolerance. I had an issue with a board that would not come out of Reset because the supervisory chip was holding the part in Reset. Technicians, the Production Manager, even Purchasing all believed that the supervisory chip was at fault because they could replace the chip and the circuit would come to life. A quick look at the spec. sheet showed that the pull-up resistor used for the supervisory chip was far too weak. Even after demonstrating that components once removed as failed were now operating properly, they refused to believe that the problem was not a faulty component, after all, the circuit had worked through two years of production without issue. I guess the cure would have been telling them that the pull-up resistor was a faulty component and needed to be replaced.

To be fair to the previous investigators, none of them were trouble shooters...they were parachuted into a situation which was sporadic in nature because of the two sets of tooling which could be installed in a random fashion. In Liaison Engineering, when something goes wrong you have to come up with a fix "toute suite"...management expects fast answers even 'though they look down their noses at the "Quick Fix" crowd (Liaison Engineers).

Yes, that happens often. Another example... quite common...was the disgruntled consumer who complained that a brand new set of batteries for his flashlight were no good. We changed a blown bulb for him and hel calmed down.

The rate at which the cells were produced provided very little time to troubleshoot, and policy was to make up the shortfall where suspect cells were concerned, quarantine the defects and pick up on their disposition later. For some reason management left me to my own devices and I could work at a rate that suited me instead of being under pressure.

Another variable may have been your testing the rejected parts during the operators' break time. If you had instead tried to take 'production' time to do the testing you may not have been allowed the time you needed.

That is very true. There is no way that time would have been allocated for troubleshooting during production. In fact several of the gizmos I designed for in process trouble shooting had to be carefully designed such that if production supervision called for their removal they could be removed on the fly without interruption to the line flow.

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